Why does rain keep bats grounded?

Working in Costa Rica, the researchers captured 10 bats that were then enclosed in a large outdoor flying cage. They measured the bats metabolism rate while at rest and then flying under three different conditions: dry, wet (researchers used tap water to wet down the fur and wings) and in heavy rain. The researchers found that dry bats expended 10 times more energy than while at rest, but that wet bats expended 20 times more energy.

Thinking that the possible explanation might be that the wet bat is carrying more water weight, the researchers weighed the bats, but found there was not difference in weight. There was also no difference in metabolic increase between the already wet bats and the ones getting wet while flying in the rain. Possible theories were that the bats did not fly in the rain because it interfered with flight mechanics, but they did not find evidence of that.

The researchers have put forth two different possible explanations for the bats not flying in the rain. Similar to how human sweat works to cool the body down, the rain may be having the same cooling effect to the bats causing them to increase their metabolism rates in order to stay warm. The other possible explanation is that the rain causes the bat’s fur to clump together and it may leave the bat less aerodynamic in flight, causing the increase in metabolic rate.

The other possible theory is that rain may have an effect on a bats’ echolocation system (the process where bats use sound that is bounced off of surrounding objects in order to find their way). Researchers have currently been unable to study this because of the incompatibility between water and their microphones.

More information: Rain increases the energy cost of bat flight, Biol. Lett. Published online before print May 4, 2011, doi: 10.1098/rsbl.2011.0313

Abstract
Similar to insects, birds and pterosaurs, bats have evolved powered flight. But in contrast to other flying taxa, only bats are furry. Here, we asked whether flight is impaired when bat pelage and wing membranes get wet. We studied the metabolism of short flights in Carollia sowelli, a bat that is exposed to heavy and frequent rainfall in neotropical rainforests. We expected bats to encounter higher thermoregulatory costs, or to suffer from lowered aerodynamic properties when pelage and wing membranes catch moisture. Therefore, we predicted that wet bats face higher flight costs than dry ones. We quantified the flight metabolism in three treatments: dry bats, wet bats and no rain, wet bats and rain. Dry bats showed metabolic rates predicted by allometry. However, flight metabolism increased twofold when bats were wet, or when they were additionally exposed to rain. We conclude that bats may not avoid rain only because of sensory constraints imposed by raindrops on echolocation, but also because of energetic constraints.

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